The present invention relates generally to Magnetic Resonance Imaging (MRI) systems, and more particularly, to a method and system for shielding a gradient coil for use in a MRI system.
Magnetic Resonance Imaging (MRI) is a well-known medical procedure for obtaining detailed, one, two and three-dimensional images of patients, using the methodology of nuclear magnetic resonance (NMR). MRI is well suited to the visualization of soft tissues and is primarily used for diagnosing disease pathologies and internal injuries.
Typical MRI systems include a superconducting magnet capable of producing a strong, homogenous magnetic field around a patient or portion of the patient; a radio frequency (RF) transmitter and receiver system, including transmitter and receiver coils, also surrounding or impinging upon a portion of the patient; a gradient coil system also surrounding a portion of the patient; and a computer processing/imaging system, receiving the signals from the receiver coil and processing the signals into interpretable data, such as visual images.
The superconducting magnet is used in conjunction with a gradient coil assembly, which is temporally pulsed to generate a sequence of controlled gradients in the main magnetic field during a MRI data gathering sequence. Inasmuch as the main superconducting magnet produces a homogeneous field, no spatial property varies from location to location within the space bathed by such field; therefore, no spatial information, particularly pertaining to an image, can be extracted therefrom, save by introduction of ancillary means for causing spatial (and temporal) variations in the field strength. This function is fulfilled by the above-mentioned gradient coil assembly; and it is by this means of manipulating the gradient fields that spatial information is typically encoded.
The gradient coil assembly produces undesirable magnetic fields outside the assembly; as well a required fields within the gradient bore. These fringe fields produce eddy currents to be formed within the magnetic structures leading to degradation in image quality.
In order to limit the effects of fringe fields generated by gradient coil assemblies, it is known that shield coils may be used in combination with the primary coils in order to cancel the magnetic field outside of the gradient coil. In this fashion the presence, and thereby the effects, of fringe fields can be reduced. Cylindrical gradient coils, however, with a large radial distance between primary and shield coils require a low current density on the shield coil to provide good cancellation of the magnetic field outside the gradient coil. The “ideal” current density needs to be approximated by physical coil winding in the gradient assembly. These approximations can be inaccurate in the situation where individual windings have a large number of Ampere-Turns relative to the peak Ampere-Turns of the shield coil. Gradient coils in this scenario often result in poor fringe field performance and often have negative effects on image quality.
It would, therefore, be highly desirable to have a gradient coil assembly with improved shield coil performance such that the effects of fringe field degradation may be minimized. It would additionally be highly desirable to have a shield coil assembly with improved winding arrangement for reduction of fringe field degradation.